Free space optical (FSO) communication systems enable higher bandwidth data transmission over conventional radio frequency (RF) systems that are employed in air or space. The limit to transmission range in these systems is on the order of several kilometers or less depending upon receiver sensitivity and laser transmitter power. In air or space a trade is made between the superior range of RF communication systems and the superior bandwidth of optical communication systems. There is a need to extend the range of FSO communication systems in order to take advantage of superior bandwidth at ranges approaching or exceeding that of RF communication systems.
Similarly, undersea data transmission and communication systems severely lack the bandwidth or range needed for numerous applications that are considered trivial in open air or space. For example, undersea acoustic communication systems are limited in data transmission rates to tens of kilobits per second and are plagued with high latency at modest ranges due to limits imposed by the speed of sound, Doppler shift from relative motion of transceivers, multi-path interference, etc. On the other hand, undersea radio frequency or electro-magnetic communication systems, which provide faster transmission rates, are limited to distances of less than approximately 10 m due to the rapid attenuation in electrically conductive sea water.
Optical communication technology can be used to increase the bandwidth for undersea communication systems. However, extremely high power lasers, on the order of several kilo-Watts, are required to overcome the rapid attenuation of light and the turbidity of the water in an undersea environment. This is true even for green and blue wavelengths, which experience the best transmission characteristics in any given undersea environment. Notwithstanding the technical challenges of producing this much laser power, the power supplies required to run these lasers become too large in size, weight, and power for practical deployment in many undersea vehicles. There is a need to extend the range of undersea optical (USO) communication systems in order to take advantage of their superior bandwidth at ranges approaching or exceeding that of acoustic communication systems, without consuming more power and without taking up more space.
Another problem of using optical communication technology is that it becomes difficult to maintain optical communication links between two moving undersea vehicles. Optical communication systems typically employ low divergence lasers with small spot sizes in order to concentrate as much signal on the receiver as possible. Tracking and adjustment of the pointing angle of the laser is required to compensate for relative motion and orientation between two undersea vehicles, or between an undersea vehicle and surface vehicle, which experiences additional motion due to waves.